1. Field of the Invention
The present invention relates to an electrical discharge machine (EDM) which variably controls the machining quality of the machined surface of a workpiece.
2. Description of the Background Art
FIG. 14 is a block diagram of a machining gap voltage application circuit in a known electrical discharge machine disclosed in Japanese Patent Disclosure Publication No. 4620 of 1986. This machining gap voltage application circuit generates a voltage applied to a machining gap between a workpiece and an electrode in an electrical discharge machine.
Referring to FIG. 14, the numeral 1 indicates an electrode, 2 a workpiece, 3 a first direct-current power supply having an output voltage E1, and 12 to 15 switching circuits, e.g., semiconductor switching devices.
A series connection of the semiconductor switching device 12 and a resistor 16 is connected between the anode of the first direct-current power supply 3 and the electrode 1, and a series connection of the semiconductor switching device 14 and a resistor 17 is connected between the anode of the first direct-current power supply 3 and the workpiece 2. The resistors 16 and 17 constitute a resistor circuit.
The semiconductor switching device 13 is connected between the cathode of the first direct-current power supply 3 and the electrode 1, and the semiconductor switching device 15 is connected between the cathode of the first direct-current power supply 3 and the workpiece 2.
A third direct-current power supply 7, whose output voltage is E3, forms a series connection with a semiconductor switching device 18, a resistor 19 and a diode 20, between the electrode 1 and the workpiece 2.
In this series connection, the diode 20 is installed in a direction in which a current flows from the electrode 1 to the workpiece 2.
Driving circuits 21-25 are operative to drive the semiconductor switching devices 12 to 15 and 18. A control circuit 26 provides a control signal to the driving circuits 21 to 25 for exercising the ON/OFF control of the semiconductor switching devices 12 to 15 and 18.
The output voltage E3 of the third direct-current power supply 7 is higher than the voltage E1 of the first direct-current power supply 3, and the resistance value of the resistor 19 is set to be sufficiently smaller than those of the resistors 16 and 17.
The voltage of the electrode 1 is transmitted to the control circuit 26 by a signal line 927, and the voltage between the workpiece 2 and the control circuit 26 is established by a signal line 928.
The control signal output by the control circuit 26 is transmitted to the driving circuits 21 and 24 via signal line 929 and the control signal output from the control circuit 26 to the driving circuits 22 and 23 is transmitted by line 930.
Finally, the control signal output by the control circuit 26 is transmitted to the driving circuit 25 via line 931.
The operation of the machining gap voltage application circuit shown in FIG. 14 will now be described. In FIG. 14, the control circuit 26 exercises control to operate the semiconductor switching devices 12 and 15 ON/OFF simultaneously and turn the semiconductor switching devices 13 and 14 ON/OFF simultaneously. Control circuit 26 also carries out control to operate the semiconductor switching devices 12 and 13 ON/OFF on a complementary basis and operate the semiconductor switching devices 14 and 15 ON/OFF on a complementary basis.
When the semiconductor switching devices 13 and 14 are turned ON and an electrical discharge is started with a higher voltage imposed on the workpiece 2 than on the electrode 1, i.e., a positive voltage is applied to the machining gap, the semiconductor switching device 18 is turned ON to impose the voltage E3 of the third direct-current power supply 7 between the workpiece 2 and the electrode 1, whereby a large discharge current flows between the workpiece 2 and the electrode 1.
After a predetermined time has elapsed, the control circuit 26 exercises control to turn the semiconductor switching devices 13 and 14 OFF and also turn the semiconductor switching device 18 OFF.
The workpiece 2 is machined by the discharge in a state wherein the positive voltage is applied to the machining gap when the semiconductor switching devices 13 and 14 are ON. In a state wherein a higher voltage is imposed onto the electrode 1 than onto the workpiece 2, i.e., a negative voltage is applied to the machining gap, when the semiconductor switching devices 12 and 15 are ON, the offset of an average voltage applied between the workpiece 2 and the electrode 1 is corrected in a decreasing direction, thereby reducing electrolysis and electrolytic corrosion.
The operation of the conventional machining gap voltage application circuit in a machine shown in FIG. 14 will now be described with reference to an operation flowchart given in FIG. 15 and the machining gap voltage and current waveforms shown in FIGS. 16(a) and 16(b).
When an operation START command is given to the machining gap voltage application circuit, the processing shifts from step S200 to step S201 in FIG. 15.
In the step S201, the semiconductor switching devices 12 and 15 are turned ON and the semiconductor switching devices 13, 14 and 18 are turned OFF.
In this state, the anode voltage of the first direct-current power supply 3 is imposed onto the electrode 1 via the semiconductor switching device 12 and the resistor 16, and the cathode voltage of the first direct-current power supply 3 is applied to the workpiece 2 via the semiconductor switching device 15.
The processing immediately shifts from the step S201 to step S202, where the state set in the step S201 is held for a period of time T1. The processing then progresses to step S203.
In the period of time T1 in the step S202, a voltage of -E1 is generated in the machining gap until an electrical discharge takes place, and when the discharge is initiated, a voltage of -E01 is developed, as shown in FIGS. 16(a) and 16(b).
From a time when the discharge is initiated until period T1 ends, a negative current of -Iop flows in the machining gap as shown in FIG. 16(b). The absolute value of the voltage -E01 is smaller than that of -E1.
In the step S203, the semiconductor switching devices 12, 15 and 18 are turned OFF, whereby the voltage is not applied to the machining gap. The processing then advances to step S204 immediately.
In the step S204, the state set in the step S203 is held for a period of time T2 shown in FIG. 16(a), and the processing moves on to a next step S205.
In the step S205, the semiconductor switching devices 12, 15 and 18 remain OFF and the semiconductor switching devices 13 and 14 are turned ON.
In this state, the anode voltage of the first direct-current power supply 3 is imposed onto the workpiece 2 via the semiconductor switching device 14 and the resistor 17, and the cathode voltage of the first direct-current power supply 3 is applied to the electrode 1 via the semiconductor switching device 13.
The processing then shifts from the step S205 to step S206 immediately.
In the step S206, it is determined whether or not an electrical discharge has taken place. If the discharge has not yet occurred, the processing proceeds to step S208.
In the step S208, it is judged whether or not a period of time T3 has ended after the shift from the step S204 to the step S205 has been performed. If it has not yet ended, the processing returns to the step S206. If the discharge has already taken place in the step S206, the processing progresses to a next step S207.
In the step S207, the semiconductor switching device 18 is turned ON. When this switching device 18 is turned ON, the anode voltage of the third direct-current power supply 7 is imposed onto the workpiece 2 and the cathode voltage thereof onto the electrode 1 via the semiconductor switching device 18, the resistor 19 and the diode 20. Then, the processing immediately advances from the step S207 to the step S208.
If, in the step S208, it has been determined that the time T3 has not ended after the shift from the step S204 to the step S205, the processing returns to the step S206 as described above. If it has ended, the processing moves on to step S209.
In FIGS. 16(a) and 16(b), the state in the steps S205 to S208 is indicated by the time T3, wherein the voltage of E1 is developed in the machining gap until the discharge is initiated, and a voltage of E11 is generated when the discharge is started.
From when the discharge is started until the time T3 ends, a positive current Ip flows in the machining gap as shown in FIG. 16(b).
The voltage of E11 is smaller than that of E1, and the absolute value of -Iop is smaller than that of Ip.
The absolute value of -Iop is smaller than that of Ip because the resistance values of the resistors 19 and 17 are smaller than that of the resistor 16 as described above.
In the step S209, the semiconductor switching devices 13, 14 and 18 are turned OFF, whereby the voltage is not imposed to the machining gap. The processing then goes to step S210.
In the step S210, the state set in the step S209 is held for a period of time T4 shown in FIGS. 16(a) and 16(b). When this time has elapsed, the processing returns to the first step S201.
In the known machine shown in FIG. 14, the negative voltage, i.e., the voltage applied to render the voltage of the electrode 1 higher than that of the workpiece 2 is supplied by the direct-current power supply 3 which supplies the high positive voltage for starting the electrical discharge. Therefore, the negative voltage also becomes high and the discharge is also developed by the negative voltage.
The discharge due to this negative voltage is a small-energy discharge which is limited in current by the resistor 16 and continues for about several ten microseconds, posing almost no problem when the workpiece 2 is a ferrous material. In sintered materials such as carbide alloys, conductive ceramics and diamond-sintered materials, however, microcracks of approximately 10 microns in depth or width will occur, significantly degrading the machining quality of the machined surface.
The discharge resulting from the negative voltage continuing for about several ten microseconds will accelerate the consumption of the electrode 1 and cause the electrode material having melted from the electrode 1 to attach to the workpiece 2, reducing the machining quality of the machined surface.
Since the machining gap voltage application circuit of the known electrical discharge machine is arranged as described above, the voltage of the power supply which imposes the negative voltage is high and the discharge current due to the negative voltage considerably degrades the machining quality of the machined surface of the workpiece 2.
Another conventional approach is disclosed in U.S. Pat. No. 4,678,884. This patent teaches the use of an inverse voltage time adjustment circuit, which detects an average voltage applied between a wire electrode and a workpiece and outputs a pulse having a width corresponding to such average voltage, and an .inverse voltage adjustment circuit which provides an output corresponding to the holding voltage of a sample hold circuit for detecting and holding a difference between a transistor-to-resistor voltage and a reference voltage at the application of an inverse voltage for a period of the output pulse time of the inverse voltage time adjustment circuit. In this conventional design, the output of the inverse voltage adjustment circuit causes the transistor to conduct and perform class A amplification operation, whereby the peak voltage of the inverse voltage is rendered constant and an average machining voltage is zeroed.
In the above referenced patent, the peak voltage of the inverse voltage is cut because great damage to the electrode must be avoided. However, since the design in the patent was not intended to prevent the discharge from occurring at the application of a negative voltage where possible, a discharge is generated at the application of a negative voltage as in the conventional design shown in FIG. 14. When the discharge takes place, the machining gap voltage is reduced, and a large current starts to flow in the machining gap. As a result, a detrimental arc is liable to persist, and this design is found to have the same disadvantages as those of the conventional design shown in FIG. 14.
In addition, since this conventional design causes the transistor to perform class A amplification operation, the transistor may generate heat if it attempts to exercise the output control of a high-power EDM power supply.